Myostatin, also known as growth and differentiation factor-8 (GDF-8), is a member of the transforming growth factor-β (TGF-β) superfamily of secreted growth factors. Myostatin has all of the structural features common to the TGF-β family proteins: a hydrophobic amino-terminus that acts as a secretory signal, nine invariant cysteine residues, and an “RXXR” furin-type proteolytic processing site. Proteolytic cleavage of the protein gives rise to a C-terminal domain which forms a homodimer that is the biologically active form of myostatin (Thies et al., Growth Factors 2001; 18(4):251-9). Alignments of the C-terminal fragment of myostatin amino acid sequences from multiple vertebrate species reveal that the protein is highly conserved (100% identity) between human, monkey, cow, dog, mouse, rat, turkey and chicken (McPherron, et al. PNAS, 94:12457-61, 1997).
Myostatin expression is limited primarily to skeletal muscle and adipose tissue, where it has been shown to be a negative regulator of skeletal muscle development (Lee L S, Immunol Endocr Metab Agents Med. Chem. 2010; 10:183-194). In mammals, skeletal muscle appears to be the principal target tissue of myostatin, where it binds to cell-surface receptors, leading to muscle loss. Mice and cattle with genetic deficiencies in myostatin exhibit dramatic increases in skeletal muscle mass, i.e., the “double muscling” phenotype, therefore supporting the role of myostatin in suppressing muscle growth (McPherron and Lee, Proc Natl Acad Sci USA. 2003 Dec. 23; 100(26):15842-6). Muscle hypertrophy in Belgian Blue and Piedmontese cattle breeds is due to a missense mutation within the third exon of the bovine myostatin gene (Bass et. al., Domest Anim Endocrinol. 1999; 17(2-3):191-7). Transgenic overexpression of myostatin inhibitors also results in hyper-muscularity. Enhanced muscle growth in these animals is due to an increase in both cell number, or hyperplastic growth, and cell size, or hypertrophic growth, which results in larger and heavier myofibers. Increased skeletal muscle mass due to a myostatin mutation has also been reported in humans. Myostatin inhibition effectively increases skeletal muscle mass and strength, both in the postnatal period and in adults.
Increases in skeletal muscle mass and strength are also associated with metabolic adaptations which positively affect body composition, energy expenditure, glucose homeostasis and insulin requirements. Both genetic and pharmacological findings indicate that myostatin regulates energy metabolism and that its inhibition can significantly attenuate the progression of metabolic diseases, including obesity and diabetes. For example, myostatin null mice exhibit decreased body fat accumulation (McPherron & Lee, J. JCI 109:595, 2002) when compared with wild type mice of the same age. This reduction in body fat is a manifestation of reduced adipocyte number and size, implicating a significant role of myostatin in adipogenesis as well as in myogenesis. Accordingly, myostatin is a desirable target for therapeutic or prophylactic intervention for the treatment of disorders or conditions which would benefit from an increase in muscle mass, muscle strength and/or metabolism (e.g., muscular dystrophy, frailty, disuse atrophy and cachexia), disorders associated with muscle wasting (e.g., renal disease, cardiac failure or disease, and liver disease), and metabolic disorders (e.g., Type II diabetes, metabolic syndrome, obesity and osteoarthritis).
Accordingly, it would be advantageous to obtain improved fibronectin domain scaffold proteins that bind myostatin for the therapeutic treatment of, e.g., metabolic disorders, muscle wasting disorders, and muscle loss due to inactivity.